Thread: Mono Stereo
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John Rumm John Rumm is offline
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Default Mono Stereo

On 17/04/2019 13:44, The Natural Philosopher wrote:
On 17/04/2019 13:33, John Rumm wrote:
On 16/04/2019 23:24, wrote:

Does it give a better S/N ratio?


IIUC, for reasons of backward compatibility, a stereo FM transmission
encodes stereo image into a pair of sum and difference signals[1]. So
mono receivers can simply process the sum signal and get both channels
combined into a single mono channel. A stereo receiver will need to
add/subtract the difference signal from the main combined channel to
get the separate L & R channels.

The sum signal is transmitted using FM and the difference signal is
amplitude modulated onto a sub carrier shifted up from the main
carrier. (IIRC there is also a pilot tone included just above the
baseband audio to signal the received that its a stereo transmission).
Since the AM modulation will suffer more in poor reception conditions
it can also introduce hiss. Many radios hence include a Stereo/Mono
switch to elect for mono with no hiss. Some of the posher car radios
actually use a mixer for the stereo decoding, so they can switch the
stereo in and out in gradual way depending on how much noise is being
detected.


[1] Conceptually not unlike the way colour was added to mono TV

Not quite correct.
https://en.wikipedia.org/wiki/FM_broadcasting#Stereo_FM

is definitive.


...and actually appears to be a fairly close match to my overview above.

the reason for worsening S/N is quite simply that more bandwidth is
needed to get stereo and that simply lets in more noise.


From your source...

"The (L+R) Main channel signal is transmitted as baseband audio limited
to the range of 30 Hz to 15 kHz. The (Lˆ’R) signal is amplitude modulated
onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal
occupying the baseband range of 23 to 53 kHz."

later:

"for a given RF level at the receiver, the signal-to-noise ratio and
multipath distortion for the stereo signal will be worse than for the
mono receiver."

AM or FM doesn't really make much odds here.


Its the AM modulated difference signal (i.e. the bit that carries the
stereo information) that suffers the poorer SNR. Whether one argues that
is because or its modulation technique or its the lower bandwidth is a
bit moot (I would say both are a factor, but AM is still the better
choice for narrowband applications).

However I will concede a better wording may have been "Since its the AM
modulated part of the signal that will suffer more in poor reception
conditions..."



--
Cheers,

John.

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